When you roll, one tank goes up, the other down. Center of gravity is unaffected.
If the tanks are a bit loose, just grab them and spin for fun
And, oh, you can actually bend you back a little, were you going to make some underwater somersaults. At least with the harness I use.
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I used a sidemount rig at a Project AWARE dive in a local river. Visibility was zero inch and the river was full of stones, bicycles, traffic signs, pipes and other stuff. I used the pull and pull -method while slowly combing the bottom, in current. Pulling loose the bicycle wrecks required some cosiderable force at times, and surfacing with one was a little awkward. I even climbed aluminum stairs, in SM rig, with fins, forcibly dragging those bicycles. The sidemount config worked well and I loved the fact that I could kneel on the bottom, bend forward, then pull and bend backward.
Haven't done this with backmount, so I cannot compare.
